Biomechanics has taught that the running gait cycle begins with the heel strike, that is, when the foot first impacts the ground. The foot first strikes the ground along a lateral portion of the heel in a supinated position. As the gait cycle continues, the foot rotates substantially transverse or inward through the midstance position toward the medial portion of the foot. When the foot moves to the medial portion, it is in the pronated phase. The final phase of the gait is the return of the foot to the center position as the runner pushes off.
There have been several inventions in recent years relating to the lateral side of the midsole in an attempt to assist runners and other athletes in a variety of manners. For example, there are a series of invention relating to methods to mitigate the likelihood of overpronation. Some of these inventions include varying the material compressibility on the lateral side in comparison to the material in the center and medial portions of the midsole. Other structures to lessen overpronation include the use of wedges to absorb the impact of the initial strike to slow the foot down as it begins to move transversely.
In addition, there have been attempts over the years to develop energy return systems and stabilizers in the center portion of the midsole. U.S. Pat. No. 5,070,629, issued Dec. 10, 1991, describes a sweet spot sole construction comprising a plurality of fibers forming a net under tension. The purpose of this system helps support the heel portion of the foot and provides energy return features as the runner complete the cycle and begins to push off.
After extended use, particularly after long distance running, the midsole region of the shoe begins to lose its shape and resiliency thus becoming less stable for the user. In fact, after repeated strikes, the midsole may become deformed into a pattern reflecting the wearer's particular gait. The forces on the midsole by the foot are not only downward, but also have horizontal components. Without any control on the midsole, other than the resiliency of the midsole material, the midsole will lose its shape over time.
Most often, the only solution for a deformed midsole is to simply throw out the sneakers and buy a new pair. Runners generally find that a pair of sneakers has a mileage limitation such as, for example, 500-1000 miles, after which the runner discards the sneakers for a new pair. When the sneakers reach the mileage limitation, the sneakers tend to no longer provide the support or resiliency necessary for top performance. The shoes are also no longer comfortable and the impact with the ground during each strike becomes more and more noticeable. The source of the mileage limitation is often the functional lifetime of the midsole.
There are several problems with this arrangement. First, top quality running sneakers are not inexpensive. Replacing sneakers every three or four months can be both costly and frustrating as the runner attempts to fine a new satisfactory pair. In addition, the runner or athlete often develops a particular fondness for his or her pair of sneakers after spending time "breaking in" other components of the shoe such as the vamp and other parts of the upper. For aesthetic reasons as well, people become attached to a particular pair of sneakers on the one hand while it is not unusual for a manufacturer to discontinue a style after one or two years on the market.
Increasing the effective functional life of a pair of sneakers is also an important factor when sneakers are used for competitive purposes. For a runner in a long distance competition, the need for a comfortable upper as well as a properly functioning midsole is important.